1. Field of the Invention
The present invention relates to an antenna, particularly a compact antenna suitable for inclusion in various devices having capabilities for processing radio signals, including various communication devices that can transmit and receive radio signals.
2. Description of the Related Art
In recent years, there have been increasing uses for antennas that can be used in frequency bands in a range of several hundreds of MHz to several tens of GHz due to increasing demand for various devices having capabilities for transmitting and receiving radio signals, including various communication devices for processing radio signals. Obvious uses for such antennas include mobile communications, next generation traffic management systems, non-contacting type cards for automatic toll collection systems, but in addition, because of the trend toward the use of wireless data handling systems that enable to handle data, without using cumbersome lengthy cables, such as cordless operation of household appliances through the Internet, Intranet radio LAN, Bluetooth and the like, it is anticipated that the use of such antennas will also be widespread in similar fields. Furthermore, such antennas are used in various systems for wireless data handling from various terminals, and the demand is also increasing for applications in telemetering for monitoring information on water pipes, natural gas pipelines and other safety management systems and POS (point-of-sale) terminals in financial systems. Other applications are beginning to emerge over a wide field of commerce including household appliances such as TV that can be made portable by satellite broadcasting as well as vending machines.
To date, such antennas described above used in various devices having capabilities for receiving and transmitting radio signals are mainly monopole antennas attached to the casing of a device. Also known are helical antennas that protrude slightly to the exterior of the casing.
However, in the case of monopole antennas, it is necessary to extend the structure for each use of the device to make the operation cumbersome, and, there is a further problem that the extended portion is susceptible to breaking. Also, in the case of the helical antennas, because a hollow coil that serves as the antenna main body is embedded in a covering material such as polymer resin for protection, the size of device tends to increase if it is mounted on the outside the casing and it is difficult to avoid the problem that the aesthetics suffers. Nevertheless, reducing the size of the antenna leads only to lowering of signal gain, which inevitably leads to increasing the circuit size for processing radio signals to result in significantly higher power consumption and a need for increasing the size of the battery, and ultimately leading back to the problem that the overall size of the device cannot be reduced.
On the other hand, when attempts are made to realize a high gain compact antenna comprised by a resonant circuit having an inductance section and a capacitance section to transmit and receive radio waves, antenna gain is affected by the environment in which the antenna in mounted such as effects from the casing of the device, and especially, if a grounded metal plate is nearby, it does not function as antenna.
The present invention is provided to resolve the problems described above in an antenna that enables to produce high antenna gain when incorporated into a device and to eliminate adverse effects of environment in which the antenna is mounted such as effects from grounded metal plates.
A first aspect of the antenna of the present invention relates to an antenna comprising an antenna main body that resonates at a center frequency and a grounding line section, connected to a ground-side of a feed line, for supplying power to the antenna main body, and emitting radio waves at the center frequency, wherein the grounding line section has a conductor portion that extends from a start terminal at which the grounding line section is connected to the feed line to a first end terminal.
By having such a structure, the antenna main body and the grounding line section floated from the surrounding ground works cooperatively to transmit or receive radio waves so that the antenna gain is improved. It is preferable that the grounding line section is formed at some distance from the antenna main body so as to prevent shorting caused by the current flowing through the capacitance existing between the antenna main body and the grounding line section. This distance of separation depends on the center frequency used for radio waves transmission and reception, but at least 10 mm is required around 450 MHz to prevent lowering the gain.
A second aspect of the antenna relates to the antenna described in aspect one, wherein a length of the conductor portion extending from the start terminal of the grounding line section to the first end terminal of the grounding line section is one quarter or an integral multiple of one quarter of the wavelength of a radio wave at the center frequency.
By having such a structure, the grounding line section is made to resonate in fixed phase so that the node of the waves always coincides with the start terminal of the shorted grounding line section, the antenna gain is improved. The length of the conductor portion between the start terminal and the first end terminal should be an integral multiple of one quarter of a wavelength of the center frequency used for transmitting and receiving radio waves through the antenna, and it is most preferable that this length is one quarter or one half of the wavelength. In this case, the longer the length of the grounding line section the higher the gain. Of course, to make the antenna smaller, it is preferable that the length of the grounding line section is one quarter of the wavelength. Although gain is not sufficiently high, similar results are obtained when the length between the start terminal of the grounding line section and the first end terminal is one eighth of the wavelength of the radio wave at the center frequency.
Also, the present invention relates to the antenna in the second aspect, wherein an impedance matching section for matching impedance values is provided between the feed end of the antenna main body and the feed line; and the impedance matching section has a matching inductance section; such that the ends of the matching inductance section are, respectively, connected electrically to the feed end of the antenna main body and to a midpoint between the start terminal of the grounding line section and the first end terminal, or it is connected electrically to the feed end of the antenna main body and to a connection site located between the start terminal and the first end terminal of the grounding line section, in such a way that a length of a part of the grounding line section extending from the start terminal to the connection site is one eighth of a wavelength of a radio wave at the center frequency.
By having such a structure, impedance matching between the circuits in the radio wave transmission and reception system and the antenna is carried out so as not to lower the antenna gain.
A third aspect of the invention relates to the antenna in the second aspect, wherein the grounding line section further has a conductor portion formed by extending from the start terminal to a second end terminal that is distanced from the first end terminal.
In this case, as a fourth aspect of the invention, it is preferable that a frequency adjusting capacitance section is provided between an exit end of the antenna main body opposite to the feed point of the antenna main body and the second end terminal of the grounding line section for adjusting the center frequency.
Further, a fifth aspect of the invention relates to the antenna in the fourth aspect, wherein a length of the conductor portion extending from the start terminal of the grounding line section to the second end terminal of the grounding line section is one eighth of the wavelength of a radio wave at the center frequency.
By having such a structure, higher gains can be achieved compared with the case of an antenna having only the conductor portion extending from the start terminal of the grounding line section to the first end terminal. Further, it enables to adjust the center frequency used for transmission and reception of radio waves so as not to lower the antenna gain.
In this case, it is preferable that the grounding line section is provided so that the conductor portion formed by extending from the start terminal to the first end terminal and the conductor portion formed by extending from the start terminal to the second end terminal surround the antenna main body, so that the first end terminal and the second end terminal are opposite to each other in such a way that these conductor portions form a loop shape having an opening at the first end terminal and at the second end terminal.
By having such a structure, a portion of the grounding line section is severed to provide the end terminal, and because it does not form a ring, the electromagnetic field from the antenna is released to the surrounding without causing eddy current to form in the grounding line section.
A sixth aspect of the invention relates to the antenna in the fifth aspect, wherein it is preferable that the grounding line section is comprised by a conductor pattern fabricated on a substrate.
By having such a structure, the grounding line section is formed on an insulated substrate to enable it to be handled as one unit when assembling the antenna into various devices having capabilities for transmitting and receiving radio waves.
Further, a seventh aspect of the invention relates to the antenna in the sixth aspect, wherein the antenna main body is constructed so that a plurality of resonance sections, each having an inductance section and a capacitance section connected electrically in parallel, are connected electrically in series so as to resonate at the center frequency.
By having such a structure, because the antenna main body is made compact by integrated circuits, assembling of the antenna into various devices having capabilities for transmitting and receiving radio waves is facilitated.
Additionally, the inductance section and the capacitance section are comprised by a plurality of conductor sections formed on a plurality of laminated substrate plates, and it is preferable that the plurality of substrate plates be formed as one unit.
By having such a structure, because the antenna main body is constructed as one unit comprised by laminating a plurality of substrate plates, assembling of the antenna into various devices having capabilities for transmitting and receiving radio waves is facilitated.
Further, it is preferable that the antenna main body is mounted on a substrate body for integrating the antenna main body with the substrate body.
By having such a structure, the antenna main body and a substrate formed with the grounding line section can be handled as one unit, thereby facilitating assembly of the antenna into various devices having capabilities for transmitting and receiving radio waves.
Beneficial effects of the antenna of the present invention are summarized in the following.
According to the present invention, the antenna is provided with an antenna main body and a grounding line section, connected to the ground-side of the feed line, for supplying power to the antenna main body in such a way that the grounding line section has a conductor portion extending from the start terminal to the end terminal, and therefore, radio waves are transmitted or received by the cooperative action of the antenna main body and the grounding line section floating from the surrounding ground, and therefore, the antenna gain is improved.
Also, according to the present invention, because the length from the start terminal to the end terminal of the grounding line section is made equal to one quarter of the wavelength of the center frequency of radio waves or its integral multiple value, so that the grounding line section is resonated and the phases of the resonating waves are fixed in such a way that the node of the waves always coincides with the start terminal of the grounding line section to be grounded, thereby increasing the gain.
Also, according to the present invention, an impedance matching section is provided between the feed end of the antenna main body and the feed line for matching impedance values, and the impedance matching section has a matching inductance section, such that the ends of the matching inductance section are connected electrically to the feed end of the antenna main body and to a midpoint between the start terminal of the grounding line section and the first end terminal, respectively, so that impedance matching between the radio wave processing system circuitry and the antenna can be carried out so as not to lower the antenna gain.
Also, according to the present invention, an impedance matching section is provided between the feed end of the antenna main body and the feed line for matching impedance values, and the impedance matching section has an matching inductance section, and the ends of the matching inductance section are, respectively, connected electrically to the feed end of the antenna main body and to a connection site that is separated from a start terminal of the grounding line section at a distance equal to one eighth of the wavelength of a radio wave at the center frequency so that impedance matching between the radio wave processing system circuitry and the antenna can be carried out so as not to lower the antenna gain.
Also, according to the present invention, the grounding line section further has a conductor portion that extends from the start terminal to the second end terminal so that the effects due to surrounding environment can be reduced further, and the antenna can be assembled into devices without lowering the antenna gain.
Also, according to the present invention, because a frequency adjusting capacitance section for adjusting the center frequency is provided between an exit end, which is opposite to the feed point of the antenna main body, and the second end terminal of the grounding line section, adjustment of the center frequency can be carried out so as not to lower the antenna gain.
Also, according to the present invention, because the length of the conductor portion extending from the start terminal to the end terminal of the grounding line section is made equal to one eighth of the wavelength of a radio wave at the center frequency, relatively high gain can be obtained compared with an antenna having only a conductor portion that extends from the start terminal connected to the feed line to the first end terminal.
Also, according to the present invention, because the grounding line section is provided in such a way that the conductor portion formed by extending from the start terminal to the first end terminal and the conductor portion formed by extending from the start terminal to the second end terminal surround the antenna main body, and that the first end terminal and the second end terminal are opposite to each other so that these conductor portions are formed in a loop shape having an opening at the first end terminal and at the second end terminal, the electromagnetic energy from the antenna can be released to the surrounding without causing eddy current inside the grounding line section.
Also, according to the present invention, because the grounding line section is comprised by conductor patterns formed on respective substrates, the antenna can be assembled easily into various devices having radio wave communication capabilities.
Also, according to the present invention, because the antenna main body is comprised by an inductance section and a capacitance section connected electrically in parallel, and a plurality of these resonance sections are connected electrically in series so as to resonate at the center frequency, the antenna can be made compact so that the antenna can be assembled easily into various devices having radio wave communication capabilities.
Also, according to the present invention, because the inductance section and the capacitance section are comprised by a plurality of conductor sections formed on a plurality of laminated substrate plates, and the plurality of substrate plates are formed as one unit, so that the antenna can be assembled easily into various devices having radio wave communication capabilities.
Also, according to the present invention, because the antenna main body is mounted on a substrate body so as to produce one antenna unit by integrating the antenna main body with the substrate body, the antenna can be assembled easily into various devices having radio wave communication capabilities.